Client should send ping

[bridger]

Amazon's Application Load Balancers are configured to shut down connections that haven't had traffic.

Sending a ping on the connection every so often is enough to keep the connection alive. I plan to implement this functionality on the WebSocket class and will submit a PR.

[tanner0101]

@bridger thanks. We automatically return a pong already for the server case but it looks like the client case needs to be sending ping at some interval. eventLoop.scheduledTask will probably work well for this.

[bridger]

@tanner0101 The scheduleTask API is what I used to address this issue in https://github.com/vapor/websocket-kit/pull/68

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